Vacuum processing chambers are generally used for chemical vapor depositing (CVD) of materials on substrates by supplying process gas to the vacuum chamber and applying a radio frequency (RF) field to the gas. A number of gas distribution systems for integrated circuit processing are known, but the vast majority of known systems are designed for low-density, high pressure plasma etching or for plasma enhanced CVD (PECVD). Conventional gas distribution systems typically deliver reactants at relatively low flow rates. Showerhead gas injection and diffusive transport systems are commonly used to ensure even distribution over the substrate.
Various systems have been proposed to inject process gas above the substrate to maximize deposition and etch rate and to minimize film deposits on various internal surfaces of the reactor. FIG. 1a illustrates a conventional plasma processing system in which gas inlets supply gas into a plasma processing chamber. As shown in FIG. 1a, this system includes a plasma source 110 for generating a plasma in a processing chamber 140 and a gas ring 167 with attached gas inlets supplying process gas into the processing chamber 140 for processing a substrate 120 on a substrate support 130. This type of system may also include an additional gas ring 160 for delivering process gas.
Other gas injection systems have been proposed to provide a more uniform distribution of process gas across a substrate surface. For example, U.S. Pat. No. 4,996,077 to Moslehi et al. discloses an electron cyclotron resonance (ECR) device including gas injectors arranged around the periphery of a substrate to provide a uniform distribution of non-plasma gases. U.S. Pat. No. 5,614,055 to Fairbairn et al. discloses a showerhead injection system in which injector orifices, located on the ceiling of the reactor, inject a reactant gas in such a manner that the reactant gas is uniformly distributed.
While these proposed gas injection systems offer improved uniformity of gas distribution above the substrate, this does not necessarily result in uniform etching or deposition. A problem with injecting gas to provide uniform gas distribution is that it is difficult to control an increase in the etch rate near the edge of the substrate and the edge of the substrate is typically etched at a faster rate than the rest of the substrate. This is referred to as "edge fast etching." This behavior adversely affects the uniformity of aluminum etching, for example, which typically has been limited to about 15% (3 .sigma.).
It has been proposed that the aluminum etch non-uniformity in Cl.sub.2 /BCl.sub.3 plasma is caused by a radial gradient in etchant concentration. This is due to the rapid reaction rate causing the etching to shift to the mass-transport limited regime. A "bulls-eye" pattern is observed in chemical driven etch systems due to more efficient replenishment of reactive species at the edges of the substrate compared to the center. This is illustrated in FIG. 1b which shows a sectional view of a typical processing chamber 140 and a computer simulated etch by-product concentration 125 above a substrate 120. The concentration of by-products is lowest near the feed gas inlet and increases towards the substrate. In FIG. 1b, the etch by-product concentration 125 is uniform across the substrate 120 except at the edge of the substrate, where it is lower due to diffusion. This leads to an edge fast etch.
To improve etching uniformity, systems have been proposed which use focus rings to "balance" gas flow above the substrate. For example, U.S. Pat. No. 5,552,124 to Y. J. Su discloses a plasma processing device including a slotted focus ring. Such focus rings are sometimes referred to as diffusion barriers, because they inhibit diffusive transport/exchange of gaseous reactants and byproducts near the substrate perimeter. That is, the diffusion barrier inhibits higher gas flow/higher plasma density at the substrate edge to avoid non-uniform processing of the substrate.
One problem with systems employing focus rings is that polymers generated from gaseous etch by-products or reactants are sometimes deposited on the focus rings. During subsequent substrate processing, this deposited polymer can cause undesirable contamination of the substrate being processed.
Other proposed systems have made use of various etchable sacrificial rings (composed of, for example, silicon or aluminum) positioned around the perimeter of the substrate to deplete the reactant gas concentration near the substrate edge. The use of sacrificial rings around the substrate is problematic because it is sometimes difficult to remove native oxides naturally formed on these surfaces, since the ion bombardment on these rings is typically less than the ion bombardment on the substrate. In addition, polymer deposition may also occur on these sacrificial rings due to the lower ion bombardment. These polymer deposits and native oxides can lead to process shift (i.e., failure to meet tolerances during sequential processing of individual substrates), which makes the sacrificial rings generally ineffective in improving the substrate etch uniformity.
Other gas supplying arrangements for plasma processing reactors are disclosed in U.S. Pat. Nos. 4,614,639; 4,996,077; 5,134,965; 5,415,728; and 5,498,313. Such reactors include low density plasma parallel plate etching reactors and high density plasma electron cyclotron resonance (ECR) microwave reactors for chemical vapor deposition (CVD) and etching. The parallel plate reactors are limited in their ability to process fine structure of semiconductor devices due to damage caused by energy of ions colliding with the substrate due to increases in applied energy and particle problems due to ion collisions with reactor walls. The ECR reactors are limited in their ability to scale to larger substrate sizes such as flat panel displays.
There is thus a need for a scalable plasma processing system which provides uniform plasma processing (e.g., etching and deposition) across the surface of a substrate but without presenting the contamination problems inherent in focus/sacrificial ring systems, or in parallel plate reactor systems.